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I observed it while trying to jump from my bed with my knees straightened and I failed to do it. I want to ask whether it is with all of us our it may be a medical issue with me?

Why we can't jump without bending our knees? I thought that the answer may be a biological one but I asked a doctor who suggested me to continue reading classical mechanics.

EDIT

I got something on this topic below:-

According to wikitionary jumping is "To propel oneself rapidly upward, downward and/or in any horizontal direction such that momentum causes the body to become airborne."

As I already mentioned that this question may seem as off topic but I think that this problem seriously involves physics!

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    $\begingroup$ You can jump with your ankles alone, but you won’t get very high. $\endgroup$
    – Gilbert
    Commented Jun 15, 2019 at 14:42
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    $\begingroup$ @Unique Gilbert is kind of correct, but I would hesitate to call it a "jump". You can exercise you calf muscles by elevating the back and arch of your foot. No bending of the knees is needed. If you elevate very quickly I suppose you might leave the ground by a very little amount and very briefly. But in my view, that's hardly a jump. So in my opinion, your question is still valid. $\endgroup$
    – Bob D
    Commented Jun 15, 2019 at 15:00
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    $\begingroup$ I'm voting to close this question as off-topic because it's really a biology/physiology question rather than a physics one. $\endgroup$
    – Kyle Kanos
    Commented Jun 15, 2019 at 15:21
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    $\begingroup$ @KyleKanos Not sure I agree. my2cts answer, which I agree with, does not invoke any element of biology or physiology that I can see. $\endgroup$
    – Bob D
    Commented Jun 15, 2019 at 16:16
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    $\begingroup$ I was able to perform very small 'jumps' with straight knees using mainly the muscles of the feet, I believe they are called toe jumps, like in this video: youtube.com/watch?v=uL7XlhdBroE $\endgroup$
    – Adam Latosiński
    Commented Jun 15, 2019 at 17:35

5 Answers 5

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To jump up by a height $h$ you legs need to perform work $W=mgh$. By flexing your legs and then stretching them you deliver $W=F\cdot s$. Making $s$ larger by flexing and stretching allows to do this with less force, so you can jump higher with the same musculature.

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    $\begingroup$ It means that it requires some potential energy of leg and earth system to reach the potential energy till $mgh$. $\endgroup$
    – Shreyansh Pathak
    Commented Jun 15, 2019 at 16:41
  • $\begingroup$ @Unique No, it just requires work. The work $W$ done by the legs or ankles is converted into kinetic energy $K$, which in turn is converted into potential energy $U=mgh$ of the Earth-body system (and soon after, that potential energy $U$ is converted back into kinetic energy $K$ as you fall back down again). We can set $W=U$ as done in this answer, because we assume zero energy loss everywhere in all these conversions. $\endgroup$
    – Steeven
    Commented Jun 15, 2019 at 17:19
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In principle, it's possible to jump off the ground without bending legs or ankles at all. If you're strong enough, you can leave the ground by holding weights in your hands and shoving the weights upward, then hanging on to the weights and letting them lift you (briefly) off the ground. Better yet, yank down on the weights and throw them to the ground. But you need to be very strong.

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    $\begingroup$ I think I wouldn't count that as jumping, more like "pulling myself up with the hands". $\endgroup$
    – Paŭlo Ebermann
    Commented Jun 16, 2019 at 0:44
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To jump, you have to give to your body some net upwards momentum. To do this without bending knees, first stand with straight legs and bend at the waist to as low as you can. Then straighten up as quick as you can, bringing your head up and your torso to vertical. At the same time flex your ankles to give a little extra lift. You can get a few inches off the ground quite easily, and I guess a gymnast could do better.

Your arms can add a slight effect if you start with them low, swing them up before you leave the ground, and then once in the air bring them down again.

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The physics here is mainly about momentum IMO. You need some vertical momentum. You get it by pushing against the floor. You move your body in any way which makes you push against the floor more than is needed merely to support your weight. Fast vertical motion of as much of your mass as possible is the aim.

All this amounts to answering the original question by saying it is possible to jump without bending our knees, and I would even add that this sort of rather artificial way to jump is not so very different from an ordinary jump: it achieves the same order of magnitude of result, because it gets most of your mass on the move. Merely flexing your ankles, by contrast, provides only a much smaller effect.

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To jump, or push yourself off the ground, you need to apply some kind of force on the ground, or in other words push off of it.

Our muscles work by squishing and stretching. When they squish they pull on bones and apply a force. Tense your biceps... they themselves squish and then pull your forearms, and the forearms move.

When you bend your knees, you squish and stretch certain muscles. As you push yourself of the ground certain muscles squish, certain stretch. The squishing ones pull your tibia/shin and/or feet bones etc... and they transfer the force to the ground. And you jump off.

You could jump by just squishing your calf muscles and pulling your feet into a "straightened" pose and push yourself of the ground that way, but that's not a lot of force, so you'd jump an inch high. However, if you squish your frontal thigh muscles, pull your shin/lower leg into a straightened position from a previously bent-knee pose, that's a lot of power and force being transferred to the ground. And you jump high.

Due to a thing called "principle of the lever" the longer the body part you move to push off, the less force you'll need to move. In simplified terms, the fulcrum here is the point of contact with the ground. Therefore, it helps to bend the knees and not just ankles, even aside from the fact most thighs are stronger than most calf muscles.

Btw, the simplest way to illustrate the principle of the lever is any door. Try pushing open a door by pushing it with your finger near the handle, at that end of the door. Easy. Now try doing it by pushing near the hinge. Yeah, good luck with that.

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You have some good answers here, explaining that there are ways to jump more or less successfully with the knees straight. However the leg and thigh muscles are generally some of the strongest in the body, this is why you get the highest jumps by bending the knees ankles and thighs, thus lowering the body, then using the powerful leg and thigh muscles to propel the body upwards quickly.

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